Field of the Invention
The invention relates to a method for controlling a fuel injector of an internal combustion engine of a vehicle, the injector comprising a piezoelectric actuator acting on a valve means to open or close the injector, thereby respectively allowing or stopping the injection of fuel into a combustion chamber of the engine, said vehicle comprising an on-board engine control unit for executing said control method.
Description of the Related Art
A piezoelectric actuator is mainly composed, in a known way, of a stack of ceramic elements defining a specified length, which has the property of having its length modified by the action of an electric field, and, conversely, of producing an electric field under the action of a mechanical stress; this stack is placed in an injector between a stop and the valve means, and operates, in summary, in the following manner: when an electrical charge is applied, by means of a voltage, to the piezoelectric actuator, its length increases and opens the valve means of the injector which thus releases fuel under pressure into the combustion chamber. More precisely, in an injection system having a high pressure common injection rail, and as a general rule, the valve means comprises a poppet actuated directly by the piezoelectric actuator, and an associated needle actuated by its contact with the high pressure in the rail, made possible by the movement of the poppet toward its open position under the action of the piezoelectric actuator. This is because the poppet, when opened, allows the high pressure from the injection rail to be put into communication with the low pressure of the return circuit to the fuel tank, thus changing the balance of forces at the terminals of the injector needle, enabling the latter to move upwards. As a result of this upward movement, the needle frees the openings of the injector jet, enabling fuel to be injected into the combustion chamber under the action of the high pressure from the rail. At rest, that is to say in a closed position of the valve means (with the poppet and needle closed), there is play between the piezoelectric actuator and the valve means, or more precisely between the piezoelectric actuator and the poppet, in order to enable this valve means to be closed and prevent uncontrolled leaks of fuel toward the combustion chamber. This play will be referred to in the rest of the present description either by its full name or by the abbreviated name of “actuator play”.
In order to be stable and have reproducible behavior, a piezoelectric actuator must be polarized at a reference value, this being done in the factory during the manufacture of said actuator, and before the engine is put into use in a vehicle. This polarization, called the initial polarization, consists in applying an electrical charge by means of a specified voltage, called the polarization voltage, for a period which is also specified, across the terminals of the piezoelectric actuator, thereby causing the crystal structure of the latter to be orientated in the direction of the electric field established in the piezoelectric stack, corresponding to the direction in which the variation of the dimension of the piezoelectric actuator is desired. After the removal of this initial polarization voltage at the terminals of the piezoelectric stack, this stack retains a residual polarization state for its subsequent use.
However, a piezoelectric actuator tends to lose this initial polarization during its use in an internal combustion engine, notably because of the essentially urban use of the vehicle, resulting in low engine speeds and therefore low nominal voltages, considerably less than the polarization voltage, for commanding the injectors under low fuel pressure. In fact, the nominal supply or command voltage applied to a piezoelectric actuator to open an injector is adjusted as a function of the requested torque and the engine speed. In particular, it is adjusted, where appropriate, as a function of the fuel pressure which is opposed to the opening of the valve means of the injector, and more generally to the energy required to open the valve means of the injector. This adjustment of the electrical charge supplied to the piezoelectric actuator, by means of the voltage for example, is thus optimized, notably, on the basis of the resistive force opposed by the fuel pressure, thereby avoiding, or tending to reduce, rattling of the injector caused by the application of a valve means opening force which is much greater than this resistive force.
It should be noted that injectors, notably for internal combustion engines running on diesel fuel at high pressure, are preferably designed so that the fuel pressure is used in such a way that it is applied, in the closed position of the injector, from the side of the valve means which keeps the latter in a position in which it bears on its seat. Additionally, the electrical charge required for the injector opening operation may be adjusted, preventing the noise generated by this opening of the injectors from being audible above the general engine noise; that is to say, a map of the nominal opening voltages of the injectors is drawn up in the factory on the basis of the engine parameters.
Other conditions of use, such as repeated cycles of increase and/or decrease in the engine temperature, or long periods without the use of the piezoelectric actuator, corresponding to prolonged periods of immobilization of the vehicle, may also lead to a change in the initial polarization of the piezoelectric actuator over time.
The depolarization, or drift, causes a contraction of the stack of ceramic elements of the piezoelectric actuator, and a consequent increase in the play between the piezoelectric actuator and the valve means. The increase in this play results in a less precise control of the actuator, or drift, which may even lead to the loss of one or more injections of small amounts of fuel, for example what are known as pilot injections, since the injector no longer has time to compensate for the clearance and open the valve for small time intervals, causing increased engine noise in the form of knocking (the main injection combustion takes place with a high pressure gradient owing to the lack of a pilot injection) and pollution, as well as making driving disagreeable. For longer periods of opening of the injector, the drift of the piezoelectric actuator leads to poor control of the amount of fuel actually injected into the combustion chamber.
The known document DE 10 2010 021448 A1 relates to a method of regulating the polarization of a piezoelectric actuator of an injector. In this document, with the aim of improving the control of the quantity of fuel injected, an output signal is superimposed on an offset voltage of the piezoelectric actuator, this offset voltage not opening the injector in any circumstances.
In French Patent Application FR 1254719, filed on 23 May 2012, not published at the date of filing of the present patent application, the applicant proposed a solution to overcome the above drawbacks. This solution consists in a method for controlling at least one piezoelectric actuator of a fuel injector of an internal combustion engine of a vehicle, said at least one piezoelectric actuator acting on a valve means to open or close said injector, thereby respectively enabling or preventing the injection of fuel into a combustion chamber of the engine, comprising the following steps:                applying a first nominal electrical charge to the piezoelectric actuator, this charge being required to open the injector, and being referred to as the nominal command charge, on the basis of the torque requested and the engine speed, in order to open the valve means of the injector to inject fuel into the combustion chamber,        commanding the closure of the injector so as to stop the fuel injection, by applying an electrical discharge to the piezoelectric actuator in order to close the valve means,said control method being applied from an engine control unit located on board the vehicle in operation, and further comprising a step of applying to the piezoelectric actuator on top of said nominal command charge, after the application of the latter and before the step of commanding a closure of the injector, at least a second electrical charge, called the polarization charge, which is additional to said nominal command charge, in order to polarize the piezoelectric actuator during an opening phase of the injector and during the injection of the fuel into the combustion chamber.        
Using this method it is possible, notably, to maintain the polarization of an actuator regardless of the use of the vehicle, since this actuator is polarized during the operation of the vehicle. One result of the application of this method for controlling a piezoelectric actuator of an injector is that excessive polarization of the piezoelectric actuator may occur, exceeding the reference or initial polarization provided in the factory during the manufacture of the injector. The main drawback of this is that it may even cause the elimination of the play between the piezoelectric actuator and the valve means which enable the valve means to be closed, and may therefore make it impossible to close the valve means of the injector by increasing the resting length of the actuator taking up said play, consequently giving rise to leaks of fuel into the combustion chamber. This is because the initial polarization of an actuator (the factory polarization) is carried out according to a predetermined polarization protocol established on the basis of the actuator itself and the voltages and/or currents available for the control of the actuator by the engine control unit. This factory polarization is not necessarily maximal for any given actuator.